Magnification systems that comprise both an objective system and a zoom system are often used in digital microscopes, the zoom system imaging the image of the object to be examined microscopically directly onto an image sensing unit of the digital microscope system. The magnification is obtained as the quotient of the zoom system focal length that is set and the focal length of the objective present in the beam path. In order to achieve the highest possible magnification, a maximum focal length must be set for the zoom system, and an objective having a short focal length must be used. Conversely, for a low magnification a minimum focal length must be set via the zoom system, and an objective having the longest possible focal length must be used.
In known microscopes a maximum zoom factor, i.e. an adjustable magnification range that is as large as possible, is achieved by utilizing the zoom range to its respective limits, and correspondingly using objectives having very different focal lengths. The maximum and minimum magnifications are thus established by adapting the objectives to the zoom system.
In order to achieve the largest possible magnification range, both objectives having a very short focal length and objectives having a very long focal length must therefore be used. Objectives having very short focal lengths are disadvantageous, however, because the numerical apertures necessary for high magnifications require a complex objective design. Such objectives then usually permit only a very narrow field angle, since otherwise the optical corrections cannot be maintained. High-aperture compound objectives therefore generally do not allow for a downstream zoom system, and cause wider field angles to be cut off due to vignetting.
Conversely, the long objective focal lengths required for low magnifications require a correspondingly long distance between the objective interface and the object plane. When such objectives are introduced into the beam path it is therefore usually necessary to move the zoom system away from the object in order to achieve the required long distance to the object plane. A further disadvantage of objectives having long focal lengths is that the pupil diameter must be correspondingly large for a given object-side resolution; this results in high cost and requires objectives having large dimensions.
The use of objectives having greatly differing focal lengths furthermore has the disadvantage that the objectives also have very different parfocalizing distances, the parfocalizing distance being made up of the distance from the shoulder surface of the objective to the object plane, plus the physical length of the objective and the clear working distance. This makes parfocal implementation of the system very complex or in fact impossible.